The invention relates to power measurements, and particularly to power measurements based on voltage and current measurements taken at separate locations.
For various reasons, it is often useful to know the amount of power being drawn on a particular branch circuit. To do so, a measured RMS current draw on the branch line is sometimes multiplied by a measured RMS voltage value over one or more line cycles to calculate the commonly known apparent power. However, it may be desirable to determine a more accurate value of the real power being drawn, one that takes into account the phase of the voltage cycle when an instantaneous current measurement is taken. If the voltage and current are measured at the same time and at the same place on the power circuit, accurately determining the power is fairly straightforward.
However, measuring voltage and current at the same location presents some challenges, and typically places additional restrictions on the current measuring side circuitry that might not be present if the current measurement were taken at a different location. For example, for safety, fire prevention, and other considerations, it is desirable and often mandated to maintain a high level of galvanic isolation between energized conductors and the circuitry used to measure the voltage between conductors and the current flowing through conductors. Measuring the voltage on energized conductors in a manner that maintains a high degree of galvanic isolation between the measuring device and the device receiving the measurement is expensive, and the cost of making individual voltage measurements when a plurality of branch circuits are to be monitored can be high enough to preclude widespread use of such an arrangement. In contrast, it is typically significantly simpler to generate signals responsive to instantaneous current with high levels of galvanic isolation, at least in situations where the current is periodic with zero mean, because magnetic coupling can be used to generate the current responsive signal. A understood current transformer is one example of a commonly known device that can generate a current responsive signal, with the electrically conductive components of the current transformer isolated from the voltage on the current carrying conductor(s) via a combination of air and materials which naturally provide a high level of galvanic isolation.
In many applications, including most power distribution systems located within an individual residence, it can be assumed that the voltage is closely modeled as a sinusoid with equal magnitude and minimal phase shift throughout the distribution system. As such, it may be possible to make a single measurement of line voltage, while making multiple measurements of the individual branch currents. However, separating the current and voltage measurement locations presents difficulties, particularly with tracking the relative phase information between the voltage and current required to estimate real power.
While a number of approaches have been proposed for measuring or estimating power, there remains a need for alternative approaches, advantageously approaches that are adapted for situations where the voltage and current on a given power circuit are to be measured at separate locations, or voltage is to measured at one location with current measured at multiple other locations.